If I remove my XP completely and install vectorlinux/puppylinux,etc,can I retrieve my old files,those I saved on hard disk when I was using XP ?

Also can anyone give a small description of how hard disk is divided in linux,like in windows it is C:,D:,E:,(local disks).

Also, I cant find files that I created using windows 7,after I rebooted Ubuntu. (In another computer)

1) If you remove XP, that means you're replacing it with Linux, which means your files are gone. You'll want to back them up to an external drive so you can copy them back. Alternatively you can shrink XP, install Linux in the free space, copy the files from the XP partition to the Linux partition, then remove XP and expand the Linux partition, but that requires a lot more work.

2) They're not divided like Windows. This is one area where Linux and Windows are completely different. In Linux, your root partition is mounted in "/". EVERYTHING falls under this location...files, directories, devices, even other drives/partitions. For example, by default user accounts are placed in "/home". Now "/home" could simply be a directory called "home" that lives on "/", your root partition, in which case everything placed in "/home" will be taking up space on your root partition. Or "/home" could be the mount point for a different drive entirely, in which case everything placed in "/home" will go into the root of your other partition. The operating system doesn't care, it writes to "/home" and it goes where it goes.

This approach to filesystem management makes sharing data between disks, migrating directories between partitions, etc. incredibly easy. If your root partition starts to fill up and you notice that "/var" is the culprit, then you can simply copy the contents of "/var" onto another drive, delete the original "/var", and then mount your other drive at "/var" (Note: since "/var" is always in use, this process would need to be performed from a live CD or the like). The OS still works like normal, but that directory now lives on another drive and the space has been cleared off of your root partition.

3) We'll need more details. Did you replace Windows 7, or did you simply boot an Ubuntu live CD? If you just booted a live CD, then chances are you just need to mount the Windows partition somewhere (like /mnt or /media) to access the drive. You may need to install the ntfs supporting utility package in order to mount it, I'm not sure if the Ubuntu live CD has those pre-installed.

I'll answer the first one. If you remove XP completely you will lose your files on that hard disk (the one that XP used to inhabit). First back up all the file you want to keep onto an external drive or USB stick. Once you have installed your Linux system you can then copy them back onto the hard drive to an appropriate folder.
As for the Linux file system, here is a pretty thorough description.
We would need far more details and far more clarity about what you did exactly. What you wrote doesn't make much sense to me.
jdk

The advice will be to copy out all the files that are important to you onto an external hard drive. Then you can install any linux distribution of your choice, then copy back all the files into the hard drive of your machine with the new operating system and you are all set.